The present invention relates in general to power supply regulation and, more particularly, to a switching regulator with fault protection.
Most if not all electronic devices require a DC voltage of appropriate level for proper operation. The DC voltage is derived from an AC power source, e.g. by plugging a power supply into a wall socket. The AC voltage available at the wall socket is converted to a DC bulk voltage by a full-wave rectifier diode bridge. The DC bulk voltage is further converted to a regulated DC output voltage by a switching power supply.
The switching power supply uses a transformer, or an inductor depending on the configuration, as an energy transfer element. For example, a flyback-type power supply has a power transistor coupled to one side of the primary winding of a transformer. The power transistor turns on and off as determined by a switching regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer develops a rectified output voltage across a shunt capacitor coupled across the secondary winding as a function of the energy transfer. The voltage across the capacitor provides the DC output voltage of the switching power supply.
The DC output voltage increases and decreases inversely with the applied load. An increasing load decreases the DC output voltage and a decreasing load increases the DC output voltage. The DC output voltage, or a representation thereof, is fed back to the switching regulator circuit to allow the switching power supply to compensate for load variation. As the load increases, the DC output voltage decreases which causes the switching regulator to leave the power transistor on for a longer average period of time in order to store more energy in the magnetic field. The additional energy is transferred to the secondary winding during the off time of the power transistor to supply the increased load and re-establish the DC output voltage. As the load decreases, the DC output voltage increases which causes the switching regulator to leave the power transistor on for a shorter average period of time to store less energy in the magnetic field. The reduced energy transfer to the secondary winding during the off time of the power transistor causes the power supply to adjust to the decreased load and reduces the DC output voltage back to its steady-state value.
A typical prior art switching regulator circuit generates a pulse width modulated control signal, or a fixed frequency, fixed duty cycle control signal which is enabled or disabled for one or more cycles in response to the feedback signal. The switching regulator generates a drive signal from the control signal to turn the power transistor on and off in order to regulate the DC output voltage across the output terminals of the switching power supply.
Many switching regulators cannot detect an overload or fault condition. A fault condition occurs when the output load exceeds the maximum rating of the power converter. A fault includes a short-circuit across the output terminals of the power supply. In a fault condition, the DC output voltage drops below its average value under nominal loading. Prior art switching regulators generally interpret a drop in the DC output voltage as an indication to supply more power to the output and bring the DC output voltage back up to its nominal value. However, supplying more power into a fault, overload, or short circuit is a safety hazard and can damage the switching power supply and/or the load itself.
Another problem experienced by prior art switching power supplies involves loss of feedback. One common feedback scheme uses an opto-isolator to monitor the DC output voltage and provide feedback information to the switching regulator. The switching regulator tends to push the DC output voltage to a maximum value absent any feedback. The feedback information operates to inhibit or disable the switching regulator as necessary to maintain the DC output voltage at a regulation threshold. If the phototransistor in the opto-isolator should fail or the feedback signal is otherwise lost, then the switching power supply would continuously deliver maximum power to load. The loss of feedback information is another fault condition that can damage the power supply and/or the load.
Hence, a need exists for a switching regulator circuit which can detect a fault condition and reduce the energy transfer to the load.